In the petroleum industry, numerous agents or contaminants can cause damage to or restriction of the production process. Examples of such contaminates can be high-molecular weight polymers (e.g. polyacrylamides, carboxymethylcellulose, hydroxyethylcellulose, CMC, HPG, and Zanthan), bacteria, sulfur, iron sulfide, hydrogen sulfide and similar compounds.
These contaminants can, in some cases, occur naturally in the formation or be present from prior human interactions. For example, bacteria are commonly introduced to the formation during drilling and workover (e.g. the repair or stimulation of an existing production well) operations. Similarly, during the fracturing process, bacteria are often introduced into the wellbore and forced deep into the formation. More specifically, polymers such as CMC, HPG, Zanthan, and polyacrilomides are added to the fracturing fluid to maintain the proppant in suspension and to reduce the friction of the fluid. Bacteria entrained within this fluid penetrate deep into the formation, and once frac pressure is released, become embedded within the strata in the same manner as the proppant deployed. Additionally, the polymer can also be deposited within the formation, causing damage in its own right. Typically, conventional “breakers” are added to the fracturing fluid along with the polymer to prevent this problem, but damage to producing wells due to the incomplete destruction of the polymer remains a common occurrence.
Many bacteria are facultative, that is they can exist in both aerobic or anaerobic conditions using either molecular oxygen or other oxygen sources to support their metabolic processes. For example, under the right conditions, facultative bacteria can use sulfate as an oxygen source and respire hydrogen sulfide, which is highly toxic to humans in addition to being corrosive to steel. Additionally, in a process known in the art as Microbiologically Induced Corrosion (MIC), bacteria will attach to a substrate, such as the wall of a pipe in the wellbore, and form a “biomass” shield around them. Underneath, the bacteria metabolize the substrate (e.g. a mixture of hydrocarbon and metallic iron) and respire hydrogen sulfide, resulting in the metal becoming severely corroded in the wellbore and, eventually, pipe failure and damage to downhole equipment. The respiration and presence of hydrogen sulfide also complicates the refining and transportation process, and attenuates the economic value of the produced hydrocarbon.
The traditional methods used to address these problems has one or more drawbacks. For example, the present industry practice is to add conventional organic and inorganic biocides, such as quaternary ammonium compounds, chloramines, aldehydes, such as Gluteraldehyde, THPS and sodium hypochlorite, to fracturing fluids with other additives to control bacteria. The efficacy of these conventional biocides, however, is minimal due to the type of bacteria that typically are found in hydrocarbon-bearing formations and petroleum production environments. More particularly, only a small percentage of these bacteria, which are often found in volcanic vents, geysers, and ancient tombs, are active at any one time; the remainder of the population is present in dormant and spore states. The aforementioned conventional biocides have no, or limited, effect on dormant and spore forming bacteria. Thus, while the active bacteria are killed to some extent, the inactive bacteria survive and thrive once they reach the environmental conditions found within the formation. Additionally, these biocides become inactivated when exposed to many of the components found in petroleum production formations. And, furthermore, microorganisms build resistance to these biocides, thus limiting their utility over time.
Chlorine dioxide, on the other hand, can inactivate or kill active, dormant and spore forming microorganisms. Unlike conventional biocides, microorganisms do not build a resistance to chlorine dioxide, and it has a low residual toxicity and produces benign end products. Chlorine dioxide is therefore an efficacious biocide, however certain applications have not been possible prior to the invention. For example, although chlorine dioxide can be applied directly to well fluids (for example, fracturing water) for disinfection, it can only be applied at a low dosage to prevent degradation of polymer(s) or other drag reduction additives.
Embodiments of this invention provide for a stable chlorine dioxide precursor additive. The inventor has found that this chlorine dioxide precursor will remain stable within, for example, well fluids (e.g. a fracturing fluid) or other fluid streams or systems until it enters a zone (e.g. within a subterranean formation) that satisfies certain conditions and reaches a minimum temperature of about 100° F.-110° F. One or more embodiments of the invention, which incorporate this chlorine dioxide precursor into a fracturing fluid, thus provide an in situ method for generating and using chlorine dioxide as a polymer oxidant and downhole biocide that does not deplete or attenuate the friction-reducing components of the fracturing fluid until the chlorine dioxide precursor is dispersed into the target zone of the subterranean, hydrocarbon-bearing formation. In these embodiments, the chlorine dioxide precursor reacts with components in the subterranean formation at a certain temperature to form chlorine dioxide therein, which then acts as a polymer oxidant and downhole biocide.
The inventor has found that the embodiments of the invention provide for results that cannot be accomplished with ex situ generated halogen dioxides, such as chlorine dioxide, or other halogen dioxide precursors, such as sodium chlorite. For example, chlorine dioxide cannot be added to well fluids (e.g. fracturing fluid) at high concentrations prior to injection into the wellbore because the chlorine dioxide will prematurely oxidize the polymers and friction-control additives within the fracturing fluid. Similarly, sodium chlorite, sometimes referred to as “stabilized chlorine dioxide,” is limited in that it immediately begins to react with weak acids and other components of the fracturing fluids at ambient temperatures, thereby generating chlorine dioxide too soon, which in turn will prematurely oxidize the polymers and friction-control additives within the fracturing fluid. By contrast, embodiments of the present invention remain generally stable until exposed to a minimum temperature and reducing agents (e.g., contaminants) located within a subterranean formation or otherwise provided in a target reaction zone.
Thus, embodiments of the invention provide for, inter alia, a composition that is stable under ambient conditions within a fluid stream or system (for example, the well fluids applied during drilling, completion, workover and fracturing operations), but subsequently reacts within a target reaction zone under specified conditions to produce a halide oxide that is capable of 1) degrading polymers within the target zone (i.e. the subterranean formation); 2) reducing toxic and unwanted sulfur compounds within the target zone (i.e. the subterranean formation and hydrocarbon deposits), and 3) functioning as a biocide that kills or destroys bacteria in active, dormant and spore forms.